It'll be a small spec bump. Tim Cook said there was something awesome late next year for professional users and I don't think he was just referring to Mac Pro users.
He clarified after the keyote via email in late 2013 a updated Mac Pro was being released so there's that not the iMac.
If the iMac was going to get a small spec bump it would have happened like the small spec bump that just happened to the Mac Pro.